I came across a question : What is the order of rate of hydration in the following compounds: enter image description here

My attempt to this question was checking the stability of carbocation. As III has a possibility of 3* carbocation with dancing resonance (hence most stable) it would have the highest rate.

How do I decide between (I) and (II) as both have 2-degree carbocation possibility with dancing resonance

Carbocation formed at (I) would have 4 alpha Hydrogen whereas carbocation formed at (II) would have 3 alpha hydrogen hence according to this logic I am getting the order II<I<III.

But we can also argue that (II) will have 2 positions at which carbocation can form hence higher rate that (I). Which factor would dominate and why?

(PS: answer given in my book is I<II<III)

  • $\begingroup$ This could be a tricky one. I'm not sure if sterics wouldn't hinder delocalisation on cyclopropyl. Huh, why do q. about this are asked so much - for me it's pretty exotic. $\endgroup$
    – Mithoron
    Sep 22 at 21:45

1 Answer 1


The fact that II can form two secondary carbocations would be the most significant factor as it provides a second mechanistic pathway which would drastically speed up the reaction.

Also notable is that:

  1. In II the double bond is more electron rich by hyper conjugation and hence will more easily take a proton so the carbocation will form faster (and carbocation formation is the rate determining step)

  2. In II by forming the cation of the right carbon of the double bond, the cation benefits from having four neighbouring C-H sigma bonds to stabilise it by hyperconjugation.


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